Ultrasonic transducers have been in use for some years, and are particularly useful for medical imaging. In typical ultrasonic imaging practice, a multi-element transducer is physically pressed against the skin of a patient, or against an organ to be imaged. Pulses of high frequency electrical energy applied to the elements of the transducer cause ultrasonic energy to be transmitted into the body part to be imaged. The ultrasonic energy is reflected back toward the transducer at the interfaces between various structures in the body. After transmission of the pulses of ultrasonic energy, the transducer is operated in a "receive mode" wherein the transducer elements provide electrical signals in response to detection, of the reflected energy. These electrical signals can be used to display an image on a video terminal including visible "clues" corresponding to the shape of the interfaces, and thus to the structure within the patient's body.
The transducers employed in ultrasonic imaging instruments are a subject of intense research and development activities, as is the associated signal generation and processing equipment employed to generate the drive signals and to process the received signals, so as to provide increasingly more detailed and readily interpreted images. More particularly, it is known to use multiple-element transducers, each of the elements being individually electrically addressed, so that the timing of the drive signals applied to each of the elements can be separately controlled. In this way, for example, an ultrasonic beam emitted by a group of elements can be focused at a desired depth in the structure, or steered in a desired direction. This practice allows structures disposed some distance from the transducer to be imaged. Commonly assigned U.S. Pat. No. 5,503,152 discusses the formation of such focused beams in detail, and also discloses methods whereby three-dimensional images can be generated.
Typical ultrasonic transducer elements are piezoelectric members formed of a wide variety of ceramic and crystalline materials. Various species of lead-zirconate-titanate (PZT) ceramics are currently the most popular piezoelectric materials for ultrasonic applications. Particular PZT materials of interest includes those known as PZT-5H, PZT-5A, PZT-4, and PZT-8. Other materials generally equivalent to PZT ceramics for the purposes of this application (where not otherwise specified) include single crystal relaxors, such as so-called PZN-PT materials, and biased electrorestrictor materials including the so-called PMN-PT ferroelectric relaxor materials. Typical compositions of these materials and suitable techniques for their processing are well known to those of skill in the art.
Piezoelectric ceramic members are typically formed by casting a slurry, comprising powders of the materials of the desired ceramic composition in a liquid polymer binder, to a desired form, followed by high temperature heat treatment of the "green" product to bake out the polymer binders and the like, and sinter the powder materials to form a ceramic. Blocks or rods thus formed can then be sliced into plates and further processed. A tape casting process providing a flat "green" tape is also known; see commonly assigned U.S. Pat. No. 5,359,760. It is also generally known that piezoelectric materials can be injection molded to form green "preforms" of relatively complex shapes, which are then similarly sintered to form ceramic members. Usefully high tolerances can be held in this process.
After sintering, the piezoelectric property is imparted to the ceramic material by "poling", i.e., by application of an electric field of a few hundred volts thereto, causing the polar ceramic molecules to be aligned in a desired direction.
Such poled piezoelectric materials have the property that when a high frequency signal (typically 3.5 MHZ) is applied thereto, the material physically oscillates, emitting acoustic energy. When reflected ultrasonic energy reaches the transducer, e.g., after reflection at an interface within a patient's body, the inverse effect is observed; that is, an electric signal is generated in response to physical oscillation of the transducer by the returned energy. These signals can be processed to yield an image of the reflecting interfaces.
When used for medical imaging purposes, the transducer must be designed for efficient transmission of the ultrasonic energy into a patient's body. The art recognizes that it would in general be desirable to reduce the size of individual transducer elements, to increase the resolution possible in the images. It is also generally desirable to reduce the acoustic impedance of the individual elements to improve the coupling efficiency, i.e., to improve the efficiency of transmission of the acoustic energy from the transducer into the patient's body. See generally Saitoh et al U.S. Pat. No. 4,958,327. Reduction in acoustic impedance, which can be accomplished by providing less ceramic material in a given element, also broadens the bandwidth of the transducer, i.e., broadens the band of frequencies within in which the transducer is useful.
The art also teaches that it would be beneficial to subdivide each individually addressable transducer element into a one- or two-dimensional array of commonly excited active emitters separated from one another by a polymer matrix or the like, and also that it is desirable to provide each emitter as a stack of two or more individual piezoelectric members electrically connected in parallel but operating acoustically in series. That is, the transducer may desirably comprise a one- or two-dimensional array of individually-addressed "elements"; each element desirably comprises a composite one- or two-dimensional array of active "emitters" in a polymer matrix, which are excited simultaneously, so that the entire surface of each composite element moves at once to emit ultrasonic energy; and each emitter may comprise a stack of two or more individual piezoelectric "members". See, for example, the inventor's Ph.D. thesis, "Analysis and Development of Piezoelectric Composites for Medical Ultrasound Transducer Applications", The Pennsylvania State University (May 1991), at pp. 63 and 84; Howarth, "Active Underwater Acoustic Attenuation Coatings", Ph.D. thesis, The Pennsylvania State University, August 1991, at pp. 119-120; and Smith U.S. Pat. Nos. 5,329,496 and 5,548,564.
There are a number of reasons why it is desirable to subdivide each element into a number of individual emitters operated simultaneously, that is, in addition to reducing the overall size of the individually-addressed transducer elements to provide improved resolution in the image generated. As noted, reducing the relative volume of the ceramic material in an element of given size reduces the acoustic impedance, more closely matching that of a patient's body, so that the ultrasonic energy is transmitted into the body more efficiently, i.e., increasing the coupling efficiency. Forming each transducer element as a number of smaller emitters in a polymer matrix also reduces lateral vibration modes, i.e., the ultrasonic energy generated in response to the high frequency excitation signal is then emitted principally in the direction of the examination, that is, into the body of the patient, rather than being transmitted laterally in the plane of the array of elements comprising the transducer, resulting in interfering modes, increased cross-talk, and a loss of usable energy.
However, a countervailing consideration is that reduction in the size of individual emitters, particularly when replacing a monolithic element with a two-dimensional array of commonly-driven individual emitters in a polymer matrix, reduces their electrical capacitance and correspondingly increases their electrical impedance. Increasing the electrical impedance is undesirable because this increases the amount of electrical energy required to transmit a given amount of acoustic energy into the patient, and results in large signal losses in cables connecting the transducer to the associated processing circuitry. As noted, increasing the size of each element would lower its electrical impedance, but would reduce the resolution of the ultimate image, which is highly undesirable.
The art recognizes that the capacitance of individual emitters can be increased and their electrical impedance reduced if each emitter comprises a number of individual piezoelectric members, bonded to one another so as operate acoustically in series (so that their acoustic energy output is effectively summed) but connected electrically in parallel, to increase their capacitance. See, for example, the Ph.D. theses and the Saitoh and Smith patents discussed above. More particularly, the capacitance increases as the square of the number of individual elements connected in parallel.
In particular, the two Smith patents and the two Ph. D. theses referred to above all teach transducers wherein each element includes a number of emitters spaced from one another in two dimensions in a polymer matrix (this being referred to as a 1-3 composite), and wherein each emitter comprises a stack of individual ceramic members connected in parallel. However, the only process for making such transducers taught by these references involves stacking the individual ceramic members of the emitters; this would be far too complicated for reliable production at reasonable cost. Furthermore, stacked composites built according to the Smith patents comprise planar continuous electrodes between the layers of individual ceramic members. Typical metallic electrode materials are considerably more dense and stiff than the polymer phase and will interfere with the free vibration of the composite when the relative amount of the metal in the composite becomes too great.
A further important consideration with respect to making a multiple-element ultrasonic transducer useful for medical imaging purposes is that the transducer must be sterilized in order to be used for medical applications. Conventional processes used to sterilize ultrasonic transducers involve low temperature liquid chemical sterilants, in which the transducers must remain for several hours to comply with the protocols in effect, or exposure to gas sterilants, after which the transducer must be "degassed" for up to 24 hours. The time-consuming nature of these low-temperature sterilization protocols necessitates that several costly transducers must be maintained for each operating suite. By comparison, most surgical instruments are sterilized much more rapidly by steam autoclaving, necessarily exposing the instrument to relatively high temperatures.
Although an ultrasonic transducer that could thus be steam-sterilized would be highly desirable, the art has thus far failed to provide a steam-autoclavable ultrasonic transducer, for a number of reasons.
More specifically, piezoelectric transducer elements lose their effectiveness if they are heated beyond the so-called "Curie temperature", at which the piezoelectric material becomes totally de-poled and loses its efficiency. Partial depoling occurs at lower temperatures, and is cumulative; accordingly the transducer will lose its piezoelectric property over repeated exposure to temperatures lower than the Curie temperature. While numerous piezoelectric materials have Curie temperatures high enough to allow repetitive steam autoclaving, the most commonly used piezoelectric material, e.g. PZT-5H, does not: PZT-5H has a Curie temperature of 193.degree. C., and exhibits an unacceptable degree of depoling at the usual steam autoclaving temperature of 140.degree. C.
The design of a transducer intended for steam autoclaving is further complicated by the composite nature of the transducer. The materials of the matrix within which the individual emitters of the transducer elements are disposed must be resistant to high temperatures and humidity encountered in the autoclaving process. Furthermore, the transducer and its elements must be designed such that differential thermal expansion exhibited by the ceramic elements, the material of the matrix, the conductors by which connections are made to the other elements of the imaging system, and the other components of a complete transducer, does not cause the transducer assembly to delaminate or otherwise be destroyed by shear stresses experienced during the high temperature autoclaving process.